This invention relates to borehole logging devices of the type wherein electromagnetic energy is used for measuring properties of formations surrounding a borehole and, more particularly, to a logging apparatus and method that can be used in either wireline logging applications or logging-while-drilling applications.
In recent years, logging systems have been proposed for employing radio frequency electromagnetic energy in the range between about 10 MHz and 100 MHz to determine both the dielectric constant and the conductivity of formations surrounding a borehole. In this frequency range, dielectric constant and conductivity both have a substantial effect upon the propagation constant of electromagnetic energy propagating in the formations, so measurements of attenuation and phase can be used for solution of simultaneous equations to determine the dielectric constant and/or conductivity of formations through which the electromagnetic energy has passed. A device of this type is the deep propagation tool ("DPT"), an embodiment of which is described in U.S. Pat. No. 4,209,747. This device includes a transmitting antenna, a "close" receiver antenna pair, and a "far" receiver antenna pair. Each of the transmitter and receiver antennas are coils wound in insulating media mounted on a metal cylindrical pipe which carries wiring to and/or from the coils. The coils are wound around the central axis of the pipe. Briefly, operation of the DPT logging device involves energizing the transmitter to emit electromagnetic energy at a frequency suitable for determination of both the electrical conductivity and the electrical permittivity of the surrounding formations. A portion of the electromagnetic energy which has traveled through the formations is received at the close and far differential receiver pairs. The signals detected at the far receiver pair are used to determine the phase shift of electromagnetic energy that has passed through the formations, and the signals detected at the close receiver pair are used to determine relative attenuation of the electromagnetic energy. The phase shift and attenuation are then employed to obtain electrical permittivity and electrical conductivity of the formations.
Various techniques of logging have been proposed for use in logging-while-drilling systems. In the U.S. Pat. Nos. 3,305,771 and 3,408,561 there is disclosed a well logging technique for determining formation conductivity (or resistivity) by employing toroidal solenoid antennas to launch a low frequency (e.g. 500 Hertz) current in a conductive sub on which the antennas are mounted. In one version of the technique, the logging device is urged against the borehole wall so that a current can flow in the device between the antennas and then through the formations and back into the device. In another version, contact electrodes extend outwardly from the device and contact the borehole wall in order to provide current paths to and from the formations. In a further form of that device wherein logging-while-drilling is effected, the described current loop passes through the metal sub which now forms part of the drill string, and then through the drill bit, the formations, and then back into the drill string. In each case, the receiving antenna or antennas measure the amount of current that flows in a loop that includes the conductive body or body portion of the logging device, and the formations. The referenced type of logging devices are advantageous in that they can be employed in logging-while-drilling applications as well as in conventional wireline logging. However, the need to establish a current loop that includes the formations, such as by making direct contact with the formations, can be problematic. Also, the frequency of the electrical energy employed is necessarily limiting on the nature of the measurement and the type of information that can be obtained.
It is among the objects of the present invention to provide a logging device which can be employed as a wireline tool or as a logging-while-drilling measurement tool. It is a further object of the invention to provide a device that is capable of electromagnetic logging at frequencies that are useful for measurement of conductivity and/or dielectric constant of subsurface formations. It is another object of the invention to provide a logging device that is capable of operating over a range of different frequencies so as to permit measurement of properties of formations at different depths of investigation.